As an electron gun for an electron microscope, a thermionic source, a Schottky electron source, a cold cathode field emission source, and the like are known. Among such electron sources, the cold cathode field emission source is a monochromatic electron source with the highest brightness, and is used for an ultrahigh resolution electron microscope. The cold cathode field emission source uses a needle-shaped tungsten tip in a non-heated state to generate an electric field with high intensity, and thus generates electrons through the tunnel effect.
Typically, the energy width of an electron beam at room temperature is greater than or equal to 200 meV (half-width) even on the ideal clean surface. However, when an electron source is used at an ultralow temperature, the energy width of an electron beam can be narrowed. For example, in the case of an electron source that uses a tungsten tip, the energy width at room temperature (300 K) is about 200 meV, but the energy width at the liquid nitrogen temperature (about 77 K) is about 120 meV, and that at the liquid hydrogen temperature (about 21 K) is about 110 meV. That is, at an ultralow temperature, the energy width can be reduced to about a half that at room temperature. Consequently, chromatic aberrations of a lens generated in the optical system of the electron microscope are reduced by half. Thus, an electron beam can be narrowed to a smaller optical beam diameter.
Meanwhile, as an ion gun for an ion microscope, a gas field ionization ion source (GFIS) is known. In the gas field ionization ion source, a strong electric field is typically caused to be generated at the end portion of a needle-shaped electrode (tip) set at an ultralow temperature that is less than or equal to the liquid nitrogen temperature, as with the cold cathode field emission source. A source gas is supplied to the periphery of the tip so that the source gas is ionized by the tunnel effect of electrons between the tip and gas molecules. For the source gas, a noble gas is used. In recent years, a helium ion microscope that uses helium ions as a probe has drawn attention. As helium ions are heavier than electrons, it is possible to reduce the wavelength and reduce the diffraction aberrations. Thus, a microscope that is far superior to a scanning electron microscope (SEM) in the resolution and the depth of focus can be obtained.
Note that a voltage applied to a tip of an ion gun is typically a positive voltage, and is opposite to that of an electron gun. That is, ions are emitted at negative potentials relative to the tip, but electrons are emitted at positive potentials relative to the tip.
For a focused ion beam (FIB) apparatus, liquid metal ions typified by gallium have been conventionally used. However, when neon (Ne), argon (Ar), or the like is used instead of the liquid metal ions, it is possible to conduct a precise sputtering process with small damage.
Patent Literature 1 discloses a cleaning method that uses an ultraviolet laser to prevent adsorption of the residual gas onto an electron source in a vacuum. Patent Literature 2 discloses a technology of a microscope having a commercialized gas field ionization ion source (GFIS) mounted thereon.